"Red letter" generally means a day or event of great significance. So in the song, they are saying how significant or important every moment they spend together is.

It comes from the phrase "red-letter day". https://en.wikipedia.org/wiki/Red_letter_day

You might see if you can listen to the song in your native language for a translation of the idiom. I know my husband listened to Disney soundtracks in Spanish when he was learning.

You need to provide more context. It is not a standard phrase